The present application relates to an optical sheet stack and a liquid crystal display device, aimed at preventing deflection of sheet, reducing the number of side-components, and preventing thermal deformation, while minimizing lowering in front luminance.
Liquid crystal display devices (LCD), characterized by their possibilities of reducing power consumption and thinning as compared with those of cathode ray tubes (CRT), have a wide variation in size, and are currently adopted to small instruments such as mobile phones and digital cameras, up to large instruments such as liquid crystal television set.
The liquid crystal display devices are classified into those of transmission type, reflection type and so forth. The transmission-type liquid crystal display device has a liquid crystal display panel composed of a liquid crystal layer and a pair of transparent substrates holding it in between, and a backlight unit as an illumination light source. The backlight unit is classified into those of direct type having the light source directly under or adjacent to the liquid crystal display panel, and those of edge-light type using a light-guide plate.
In general, the backlight unit of the liquid crystal display device is configured using an optical sheet or film (simply referred to as “sheet” hereinafter) having a condensing function, such as a prism sheet, lenticular lens sheet and so forth, collimating direction of emission of light from the light source to align to a front direction. For example, the prism sheet is configured as having a large number of prism components having a triangular section arranged on a light extraction side thereof, allowing the light incident on the prism sheet to refract and transmit therethrough, so as to collimate the light to the front direction. The optical sheets other than the prism sheet include diffuser sheet having a function of diffusing light, and reflection-type polarizer having a function of polarizing light, which are used in combination with the prism sheet so as to make luminance of the liquid crystal display device uniform and intense.
On the other hand, bright-and-dark pattern (moire) due to interference of light may occur, ascribable to interaction between a pitch of arrangement of prisms on the prism sheet and pixel pitch on the liquid crystal display panel. Known methods of preventing the moire include a method of narrowing the pitch of arrangement of prism to as narrow as 100 μm or smaller, and a method of disposing a diffuser sheet between the prism sheet and the liquid crystal display panel (Japanese Patent Application Publication (KOKAI) No. Hei 6-1025061: Patent Document 1).
Alternatively, Japanese Patent Application Publication (KOKAI) No. 2004-46216 (Patent Document 2) discloses a configuration of a liquid crystal display device having a reflection-type, polarized-light separation element allowing a first linearly polarized light to transmit therethrough, and reflecting thereon a second linearly polarized light, disposed on the surface-of-incidence side or on the surface-of-extraction side of the prism sheet.
By the way, there has been a large trend of increasing screen size in the field of liquid crystal television set. With increase in the screen size, also optical sheets such as the diffuser sheet, prism sheet (or lens sheet), reflection-type polarizer sheet has been increased in size, wherein a problem has arisen in particular for the prism sheet (or lens sheet) and reflection-type polarizer sheet, having small thickness, in that they are very difficult to handle in the process of assembly. In addition, there has been much waste of side-components such as protective sheet placed on both surfaces of the optical sheet, when the optical sheet alone is transported.
Moreover, with increase in the screen size, illuminance of the light source becomes inevitably large in order to ensure a necessary level of brightness of the display surface. For this reason, also energy of heat applied to the surface having an increased area increases. Because of a large area of sheet, thus increased heat fails to conduct uniformly over the surface of sheet, so that the sheet never deforms uniformly. As a consequence, the contact between the optical sheets, or between the optical sheet and the liquid crystal display panel may occur, and thereby image quality of displayed image may degrade.
There is known a method of bonding the optical sheets to be stacked using a transparent adhesive over the entire surfaces thereof in the order of stacking, typically as described in Japanese Patent Application Publication (KOKAI) No. Hei 9-146093 (Patent Document 3). Thus-configured optical sheet stack may be improved in the rigidity by virtue of bonding of two or more optical sheets, may consequently be improved in the handleability in the process of assembly while suppressing deflection, and may be halved in consumption of the side-components. Bonding of the optical sheets may also improve the stiffness of sheet, and may make the sheet more resistant to thermal deformation.